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In rare cases, the date of coitus is known, and this may be useful in calculating the length of pregnancy.For related information, see Medscape's Pregnancy Resource Center.Therefore, giving a range for the likely date of birth (eg, EDD -2 weeks to EDD 1 week) is more useful.Infants born before 37 completed weeks' gestation are deemed preterm, whereas those born after 42 weeks' are considered post-term.The accuracy of menstrual history in women with a history of oligo-ovulation, such as those with polycystic ovarian syndrome, should be questioned.If conception occurred while oral contraceptives or long-acting progestogens were being taken, the LMP cannot be used because it has no relation to the time of ovulation.In patients who have had ovulation induction, calculate GA from the day of human chorionic gonadotropin administration.The size of the uterus can be assessed by pelvic examination or by abdominal palpation.
In order to calculate the EDD, the practitioner must know the median length of normal pregnancy and the last menstrual period (LMP) or ultrasonographic estimation of gestational age (GA).
The quality of these wheels varies, but in general, the larger wheels yield better results.
More accurate results can be obtained by using computer software or web-based online calculators (eg, as found online at the Gestation Network).
Size can be misleading in the presence of multiple pregnancy, uterine fibroids, or a full bladder.
Tape measurement of the symphysis-fundus height may be useful up to 28-30 weeks' gestation, beyond which it becomes too inaccurate for dating.